1. Field of the Invention
The present invention relates to surface treated inorganic pigment, a process for its preparation and its use in polymer compositions. More particularly this invention relates to polymer compositions containing the surface treated inorganic pigment and a polymer processing aid.
2. Description of the Related Art
The melt extrusion of polymers, for example, hydrocarbon polymers and polyamides, into shaped structures such as tubing, pipe, wire coating or film is accomplished by well-known procedures wherein a rotating screw pushes a viscous polymer melt through an extruder barrel into a die in which the polymer is shaped to the desired form, and is then subsequently cooled and solidified into a product, that is, the extrudate, having the general shape of the die. In film blowing processes, as an extruded plastic tube emerges from the die the tube is continuously inflated by air, cooled, collapsed by rolls and wound up on subsequent rolls.
Melt fracture is a flow phenomenon that occurs as the molten polymer flows through the die, starting at the die entry, evidenced by gross irregularities in the shape or surface of the extrudate. Melt fracture is considered to be the result of non-uniform or irregular elastic strains in the material at the die entrance. The shear rate at the surface of the polymer is sufficiently high that the surface of the polymer begins to fracture. That is, there is a slippage of the surface of the extruded polymer relative to the body of the polymer melt. The surface generally cannot flow fast enough to keep up with the body of the extrudate and a fracture in the melt occurs. These irregularities in the shape or surface of the extrudate are undesirable. For example, the irregularities produce an unattractive pattern on blown films. Processing aids are typically added to the polymer so that during melt processing they will migrate to the surface of the polymer lubricating the polymer and die surfaces thereby allowing high throughput with reduced melt fracture.
U.S. Pat. No. 6,593,400 discloses film forming polyolefin compositions containing antiblocking agents and polymer processing aids which exhibit reduced melt fracture. The '400 patent teaches that processing aids are adversely affected by antiblocking agents because the antiblock agent adsorbs the processing aid. The '400 patent describes an antiblock agent produced by surface treating an inorganic mineral with a functionalized siloxane polymer or a polyether polymer or functionalized polyether polymer or carbon based polymer. The carbon based polymers include maleic acid/olefin copolymers; maleic acid/styrene copolymers, mineral oils and paraffin waxes. The '400 patent teaches that such surface treated inorganic minerals used as an antiblock agent in the production of polyolefin film reduce the adsorption of processing aids which reduces melt fracture. The described inorganic mineral is selected from the group consisting of talc, calcium carbonate, precipitated calcium carbonate, clay and silica.
It is well known that antiblocking agents work by migrating to the surface of the polyolefin during melt processing to roughen the surface so that layers of the finished plastic film will not stick together. Both the antiblocking agent and the processing aid work at the surface of the polyolefin film.
Titanium dioxide pigments are added to polymers for imparting whiteness and/or opacity to the finished article. Unlike antiblocking agents and processing aids which are selected for their ability to migrate to the polymer surface, titanium dioxide pigments have not been known to show any preference for surface migration. A desirable shiny polymer surface occurs because the titanium dioxide does not have a tendency to break through the polymer surface in the typical manner of antiblock particles. Titanium dioxide pigments which have a specific gravity of about 4.3 g/cc typically become uniformly dispersed throughout the polymer.
Duchesne, et al., in Polymer Engineering and Science (1990), 30(16), 950-6, disclose the performance of a fluorocarbon elastomer processing additive in rutile-filled linear low density polyethylene compounds. Lower apparent melt viscosities and higher shear rates for the onset of melt fracture were reported for certain surface-treated rutiles. Duchesne at al. further describe a negative interaction between rutile titanium dioxide and polymer processing aids in linear low density polyethylene and propose minimizing the negative interaction by using separate masterbatches to shorten the extent and time of contact between pigment and processing aid which contact is considered to promote adsorption of the polymer processing aid onto the pigment which inhibits the function of the processing aid.
There is a significant disadvantage to using separate masterbatches. Separate masterbatches increase the cost of polymer production by adding to raw materials manufacturing, shipping and storing costs.
The present invention solves the problem of melt fracture resulting from the negative interaction between the processing aid and the inorganic pigment and permits the use of a combined pigment processing aid masterbatch. In particular, a shaped article of reduced melt fracture can now be made from a masterbatch containing both the inorganic pigment and the processing aid, and other additives, without concern for the negative interaction between the pigment and the processing aid which has been known to lead to melt fracture and other problems, such as die lip buildup.